1. Field of the Invention
This invention relates to a road surface condition-detecting system for automotive vehicles, which detects a road surface condition based on road noise generated by traveling of the automotive vehicle on a road surface, and an anti-lock brake system employing the same.
2. Prior Art
Conventionally, a road surface condition-detecting system for automotive vehicles has been proposed, which is adapted to detect a road surface condition based on road noise generated by traveling of the automotive vehicle on a road surface. As such a road surface condition-detecting system, the present assignee has already proposed one by Japanese Provisional Patent Publication (Kokai) No. 6-50878, in which road noise is detected by a microphone arranged in the interior of a vehicle body in the vicinity of a tire of a wheel, and a signal indicative of the detected road noise is subjected to frequency analysis and then passed through a bandpass filter to take out a sound pressure level of a component of the noise having a particular frequency, whereby the detected sound pressure level of the noise component is compared with a reference level stored or mapped in advance according to the vehicle speed and the road surface condition, to thereby determine the road surface condition.
Further, an anti-lock brake system (ABS) has been conventionally used to prevent locking of tires caused by a quick braking operation. This kind of system performs control of braking torque based on the wheel speed VW and estimated vehicle speed VR to prevent wheels from being locked, thereby maintaining controllability and safety of the vehicle.
Although the conventionally-proposed road surface condition-detecting system can effectively discriminate one road surface condition from others, when they are small in number, it is not suitable for discriminating one from a large number of possible road surface conditions, since there is a high possibility that such a discrimination will require an immense quantity of reference data and accordingly an increased number of particular frequency bands in which detected sound pressure levels are compared with respective reference levels, and further necessitate setting and tuning of a margin for the reference sound pressure level for comparison, according to the kind, pneumatic pressure, and wear of tires used, etc.
Further, to detect the road surface condition more accurately, it is necessary to increase the amount of data to be stored for determination of the road surface condition and reduce the sampling repetition time for data detection to thereby increase the amount of sampled data. The resulting increase in the computing time makes it very difficult to detect the road surface condition in real time.
Still further, according to the conventional road surface condition-detecting system, a road noise sensor arranged in the vehicle picks up disturbance other than the road noise, such as vibration noise of the vehicle body, an output from audio equipment, exhaust noise of the engine, which prevents the system from accurately or properly detecting or determining the road surface condition.
Moreover, in the conventional road surface condition-detecting system, the road surface condition is detected at predetermined time intervals, which may result in an erroneous determination of the road surface condition as a whole in the event that the road surface condition is not uniform.
FIG. 1 shows an example of the road surface condition detected by the conventional road surface condition-detecting system installed on a vehicle C traveling on a dry road surface pitted with puddles 101, 102. In the figure, timing for detecting the road surface condition, and results of detection or determination are depicted. The puddles 101, 102 are not so large as will require a vehicle body control e.g. by an anti-lock brake system and a traction control system, or an alarm cautioning the driver of the road surface condition, but rather negligible enough to determine that the road surface as a whole is dry.
As shown in FIG. 1, when the vehicle C encounters the puddles 101, 102 at detection timing points D, G while traveling on the dry road surface 103, the road surface condition is determined to be wet (as indicated by the symbol WET). In response to the determination, the vehicle body control, the alarm generation, etc. are carried out. However, the puddles 101, 102 are not so large and therefore the road surface condition as a whole should desirably be determined to be dry for the purpose of stable traveling of the vehicle. In the case of FIG. 1, the road surface is liable to be determined to be a different one each time the determination is carried out so that the vehicle body control, the alarm generation, etc. are carried out, resulting in degraded traveling stability of the vehicle.
Further, in the conventional anti-lock brake system, parameters for controlling the braking torque, such as a desired slip ratio of wheels and the maximum deceleration G, are set to respective fixed values irrespective of the road surface condition.
FIG. 2 shows curves of braking friction coefficient-slip ratio characteristics, in which a curve RA1 represents a characteristic exhibited when the vehicle is traveling on a dry road, while a curve RB1 represents one exhibited when the vehicle is traveling on a gravel road. The braking friction coefficient .lambda. assumes the maximum values at slip ratios .lambda.A and .lambda.B in the curves RA1 arid RB1, respectively. For example, if a brake control system in which the desired slip ratio is set to the .lambda.A carries out the braking torque control on a road surface exhibiting the characteristic of the curve RB1, the braking force can be merely obtained through a braking friction coefficient of .mu.2, failing to obtain the braking force to be achieved by the maximum braking friction coefficient of .mu.1 on the road surface having the characteristic represented by the curve RB1.
FIG. 3 and FIG. 4 show examples of the braking torque control actually carried out on a dry road and a gravel road, respectively, by the use of the conventional anti-lock brake system. In both the figures, the ordinate represents vehicle speed, while the abscissa represents time. Further, solid line curves VA1, VB1 show changes in the actual speed of the vehicle on the dry road and the gravel road, respectively, broken line curves VA2, VB2 show changes in an estimated speed of the vehicle on the dry road and the gravel road, respectively, and one-dot chain line curves VA3, VB3 show changes in a wheel speed of the vehicle on the dry road and the gravel road, respectively.
The estimated speed of the vehicle is calculated from the wheel speed, and a predetermined slip ratio, i.e. the desired slip ratio .lambda.A. Therefore, when the vehicle is traveling on the dry road, the difference between the actual vehicle speed VA1 and the estimated vehicle speed VA2 is small as shown in FIG. 3. When the vehicle is traveling on the gravel road, however, the difference between the actual vehicle speed VB1 and the estimated vehicle speed VB2 becomes large, as shown in FIG. 4.